Aerosolized spores of Bacillus anthracis represent one of the most serious bio-terrorist threats to the security of the United States. New therapeutics based upon novel chemical scaffolds are vital to the biodefense armory because they are likely to be effective against both natural and engineered resistant forms of B. anthracis. Our strategy is to build effective anthrax therapeutics not by targeting the viability of the organism but by targeting the mechanism of pathogenesis directly. Toward this aim, we have identified drug-like small molecule LF inhibitors through a combination of screening of chemical libraries and 3D sub-structure database mining. These inhibitors have been shown to specifically bind to LF with low (M potencies in enzymatic assays and to protect macrophages against challenge with toxin containing LF. The identified inhibitor classes are novel and devoid of non-drug like features such as the hydroxamic acid and peptidic backbone, which are common in many known protease inhibitors. Thus, this validated hit series forms the basis for development of efficacious, safe and orally bioavailable drugs against anthrax. To further enhance the probability of success of this project, the X-ray structure of the inhibitor bound in the active site of LF has been solved thus providing a powerful tool, which we will use to guide the inhibitor refinement process through structure-based drug design. The overall goal of this project is to develop a small molecule LF inhibitor to treat anthrax. In Phase I, we will apply proven techniques of medicinal and combinatorial chemistry; inhibitor-enzyme complex X-ray crystallography and structure-based drug design (SBDD) to rapidly synthesize and evaluate derivatives of this new validated hit series. In an iterative process, we will probe focused compound libraries for features contributing to tighter binding and more potent inhibition of LF by measuring the enzymatic, cellular activity and specificity of derivatives. We will determine the X-ray structures of improved inhibitors bound to the LF active site, and use these data to develop refined pharmacophore models to guide further probing of the structure activity relationship (SAR). In addition, we will assess compounds for optimal ADME (Absorption, Distribution, Metabolism, Elimination) properties (e.g., liver microsome stability, Cyp isoform inhibition and Caco2 permeability). Compounds with sufficient enzymatic, cellular potency and ADME properties will be synthesized at the 1-2 g level and tested for efficacy in a LF induced rat death model. Successful rescue of rats from LF-induced death will qualify compounds as in vivo-validated leads. In Phase II, we will further evaluate these leads for in vivo efficacy, pharmacokinetic properties, toxicity and safety pharmacology, in two species, in order to develop them into pre-IND clinical candidates, suitable for human clinical trials (Phase III).